1. Field of the Invention
The present invention relates to cavities and channel add/drop filters employing photonic crystals, and in particular to improvements in the characteristics of cavities and channel add/drop filters based on two-dimensional photonic crystals.
It should be understood that in the present specification, the significance of the term “light” is meant to also include electromagnetic waves that relative to visible light are of longer as well as shorter wavelength.
2. Description of the Background Art
Along with advances in wavelength division multiplexed (WDM) optical communication systems in recent years, the importance of ultrasmall add/drop filters and channel filters in which enlarged capacity is being targeted is on the rise. In this area, then, attempts are being made to develop extraordinarily small-scale optical add/drop filters by employing photonic crystals. In particular, with photonic crystals novel optical properties can be realized by exploiting artificial periodic structures in which a crystal-lattice-like periodic refractive index distribution is artificially imparted within the parent material.
One important feature of photonic crystals is the presence of photonic bandgaps. With photonic crystals having three-dimensional refractive index periodicity (3D photonic crystals), perfect bandgaps in which the transmission of light is prohibited in every direction can be formed. Among the possibilities with these crystals are the local confinement of light, control of spontaneous emission, and formation of waveguides by the introduction of line defects, wherein the realization of ultrasmall photonic integrated circuits can be anticipated.
Meanwhile, studies into uses for photonic crystals having a two-dimensional periodic refractive-index structure (2D photonic crystals), are flourishing because the crystals can be manufactured comparatively easily. A periodic refractive-index structure in 2D photonic crystals can be formed by, for example, arranging in a square-lattice or triangular-lattice geometry air rods perforating a high-refractive-index plate material (usually termed a “slab”). Alternatively the structure can be formed within a low-index material by arranging, in a 2D-lattice geometry within it, posts made of a high-refractive-index material. Photonic bandgaps can be produced from such periodic refractive-index structures, enabling the transmission of light traveling in an in-plane direction (direction parallel to both the principal faces of the slab) to be controlled. Waveguides, for instance, may be created by introducing line defects into a periodic refractive-index structure. (See, for example, Physical Review B, Vol. 62, 2000, pp. 4488–4492.)
FIG. 12 illustrates, in a schematic oblique view, a channel add/drop filter disclosed in Japanese Unexamined Pat. App. Pub. No. 2001-272555. (In the drawings in the present application, identical reference marks indicate identical or equivalent parts.) The channel add/drop filter in FIG. 12 exploits a 2D photonic crystal having, configured within a slab 1, cylindrical through-holes 2 of identical diameter (ordinarily occupied by air) formed at the vertices of a 2D triangular lattice. In a 2D photonic crystal of this sort, light is prohibited from propagating in an in-plane direction within the slab 1 by a bandgap, and in the direction normal to the plane (direction orthogonal to the two principal faces of the slab) is confined due to total internal reflection occurring at the interface with the low-refractive-index clad (air, for example).
The photonic crystal in FIG. 12 contains a waveguide 3 consisting of a straight line defect. This straight-line defect 3 includes a rectilinearly ranging plurality of lattice points adjoining each other, with the through-holes 2 missing in these lattice points. With light being able to propagate through defects in the 2D photonic crystal, the straight-line defect can be employed as a linear waveguide. With linear waveguides, the spectrum of wavelengths in which light can be transmitted at low loss is comparatively broad; consequently light in a wide range of wavelength containing signals in a plurality of channels may be propagated through them.
It will be appreciated that the width of straight-line defects as waveguides can be altered variously in accordance with the requested characteristics. The most typical waveguide is obtained, as described above, by leaving through-holes missing in one row in lattice-point line. Nevertheless, waveguides can also be created by leaving through-holes missing in a plurality of neighboring rows in the lattice-point lines. Moreover, a waveguide is not limited in width to integral multiples of the lattice constant, but may have an arbitrary width. For example, it is possible to create a waveguide having a width of choice by relatively displacing the lattice on either side of a linear waveguide to the distance of choice.
The photonic crystal set out in FIG. 12 also contains a cavity 4 consisting of a point defect. The point defect 4 contains a single lattice point, and through that lattice point a through-hole that is of large diameter as compared with the other lattice points is formed. A defect in this way containing a relatively large-diameter through-hole is generally termed an acceptor-type point defect. On the other hand, a defect in which a through-hole is missing in a lattice point is generally termed a donor-type point defect. The cavity 4 is disposed adjacent the waveguide 3, within a range in which they can exert on each other an electromagnetically reciprocal effect.
In a 2D photonic crystal such as that illustrated in FIG. 12, if light 5 containing a plurality of wavelength ranges (λ1, λ, . . . λi, . . . ) is introduced into the waveguide 3, light that has the specific wavelength corresponding to the resonant frequency of the cavity 4 is trapped in the cavity and while resonating in the interior of the point defect, light 6 of wavelength λi is emitted in the normal direction, in which the Q factor originating in the finite thickness of the slab 1 is small. This means that the photonic crystal in FIG. 12 can be employed as a channel drop filter. Conversely, by shining light into the point defect 4, in the direction normal to the slab 1, light of wavelength λi that resonates within the cavity 4 can be introduced into the waveguide 3. This means that the photonic crystal in FIG. 12 can also be employed as a channel add filter. It will be appreciated that the transfer of light between either the waveguide 3 or the cavity 4 and the exterior can be made to take place by proximately disposing an optical fiber or an optoelectronic transducer in the vicinity of the waveguide end faces or the vicinity of the cavity. Of course, in that case a collimating lens (collimator) may be inserted in between either the waveguide end face or the cavity, and the optical-fiber end face or the optoelectronic transducer.
In a an optical add/drop filter such as that illustrated in FIG. 12, by appropriately configuring the spacing between the waveguide 3 consisting of the line defect and the cavity 4 consisting of the point defect, the ratio of optical intensities in a specific wavelength that is transferred between the waveguide and the cavity can be controlled. Also in FIG. 12, since no asymmetry is introduced with respect to the point defect 4 in the direction normal to the slab 1, light is output in both vertical directions from the point defect 4; but it is possible to make the output of light be in only one or the other vertical direction by introducing asymmetry in the point defect 4 in the plane-normal direction. An example of a mechanism that can be utilized to introduce this sort of asymmetry is a method in which the diameter of the point defect 4, which is round in section, is made to vary continuously or discontinuously along the thickness of the slab. With further regard to FIG. 12, although the channel add/drop filter in the figure contains only a single cavity, it will be readily understood that by disposing along the waveguide a plurality of cavities differing from one another in resonant wavelength, optical signals in a plurality of channels can be added/dropped.
With the Q factor of a cavity employing an acceptor-type point defect such as disclosed in Japanese Unexamined-Pat. App. Pub. No. 2001-272555 being around 500, the full width at half-maximum (FWHM) in the peak-wavelength-including light output from a cavity of this sort is around 3 nm.
However, using multi-channel signals for WDM communications at about 100 GHz with a wavelength-peak spacing of approximately 0.8 nm is being investigated. This means that with a cavity such as disclosed in Unexamined Pat. App. Pub. No. 2001-272555, the largeness of the Q factor is insufficient, and with the 3-nm FWHM, the cavity is totally inadequate for separating from one another multi-channel signals whose peak-wavelength spacing is approximately 0.8 nm. In short, there is a need to raise the Q factor of cavities employing 2D photonic crystals, to reduce the FWHM of the peak-wavelength spectra they output.